With the fierce global competition and rapid technological advancements that are being made in the field of wireless communications, there is the constant need to improve product quality, to reduce product cost, and to improve performance of communication devices. One of the more costly components in a radio frequency (RF) communication device is the RF power amplifier.
A class of RF power amplifiers achieve high RF power by operating RF amplifier circuits in parallel, splitting one or more RF signals among the inputs to each of those parallel circuits. A block diagram of such an RF power amplifier configuration is shown in FIG. 1. An RF power amplifier 100 comprises a first and a second RF amplifier stage 134 and 136 configured to operate in parallel, a signal splitter 104, and a signal combiner 118. The first and second RF amplifier stages 134 and 136 each typically include an RF amplifier 110 and 112, an input matching circuit 106 and 108, and an output matching circuit 114 and 116.
The signal splitter 104 is a hybrid splitter that typically comprises transmission lines 101, 103, 105, 107, and 109, and a resistor 111. The impedance values of these transmission lines and resistor may vary depending upon the input impedance of the input matching circuits 106 and 108 and the output impedance of the circuit preceding the signal splitter 104, since there is a transformation from the former impedance to the latter impedance. Similarly, the signal combiner 118 illustrated here is a hybrid combiner that typically comprises transmission lines 120, 122, 124, 126, and 128 and a resistor 130. Again, the impedance values of these transmission lines and resistor may vary depending upon the output impedance of the output matching circuits 114 and 116 and the input impedance of the circuit succeeding the signal combiner 118.
When an RF signal 102 is applied to the RF power amplifier 100, the signal splitter 104 divides the RF signal power equally between the two RF amplifier stages 134 and 136. The signal splitter transmission line 109 is designed to be one-quarter wavelength long. As a result, the RF signal input into the first RF amplifier stage 134 is 90 degrees out of phase from the RF signal input into the second RF amplifier stage 136. Reflections by the two amplifier stages 134 and 136 of signals of equal amplitude and phase will cancel each other out when combined in the signal splitter 104, which cancellation will improve the return loss (the ratio of reflected input power to input power) performance of the RF power amplifier.
Cost reductions have been achieved by reducing the space consumed by these parallel RF power amplifier circuits 134 and 136 and placing them on a single circuit board, along with their associated signal splitters 104 and combiners 118. Further cost and quality improvements have been achieved by automating the manufacturing processes, and additional areas of automation are always being explored. One such area of automation is the tuning of the RF power amplifier circuits to optimize their RF performance. However, a problem exists with respect to the automated tuning of each of two or more parallel RF amplifier stages on the same circuit board when the RF input to each stage cannot be automatically isolated during the tuning process. If the RF input to each stage cannot be isolated, then the performance of that stage cannot be individually observed and individually tuned (optimized). The signal splitter provides power to all parallel stages and therefore measurements such as efficiency, gain, output power and return loss that are made during the tuning procedure are an aggregate of the performance of all of the stages. Furthermore, the return loss performance is a product of RF power reflecting back from all of the stages to which RF power is provided, and the use of a hybrid signal splitter will cause the reflected power from individual stages to cancel out in the splitter. Thus the individual stages could be reflecting back most of their input power, resulting in little of the input RF signal being amplified, but the RF power amplifier's return loss could appear to be very good and there would be little clue as to why the RF power amplifier's gain and output power would be particularly low. Hence there is a need for an apparatus that will optimize the tuning of amplifier circuits while overcoming the deficiencies of the prior art.